1. Field of the Invention
This invention relates to napped fluororesin materials having continuous pores, and which are used for the prosthesis of a damaged heart or other organs, or blood vessels, or the manufacture of filters, clothes, or the like.
2. Description of the Prior Art
Sheets or tubes of fluororesins having continuous pores, particularly polytetrafluoroethylene (PTFE), are widely used for making patches, artificial blood vessels, or other medical substitutes, various types of filters, diaphragms, or water-repellent, air-permeable clothes. These products are all very effective. Such PTFE material having continuous pores can be made by a method as disclosed in U.S. Pat. No. 3,953,566. According to this method, an admixture of fine PTFE powder and a liquid lubricant is preformed into a paste, the paste is extruded through a ram extruder, and the extruded product is stretched in a selected direction at a stretch rate of at least 10% per second, or its original length in the stretched direction, at a temperature lower than the melting point of PTFE crystals (about 327.degree. C.). If required, the material may be heated at a temperature higher than 327.degree. C., while it is held against shrinkage, to provide a baked product. Products having different shapes and physical properties can be obtained if the shape of the product obtained by extrusion, the stretch ratio and the heating temperature are varied. These stretched porous PTFE products comprise numerous fine nodes interconnected by fibrils, and defining numerous fine continuous pores having a diameter of, say, 0.1 to 5 microns. The tubular products may be used individually or stacked together to provide artificial blood vessels, while the membrane products may be laminated on other material to make various types of filters or clothes, or the like.
None of the known products have, however, been satisfactory for all end uses. Some materials for prosthetic use comprise expanded PTFE alone, while others comprise a combination of expanded PTFE and other woven or knitted material, or plastic coating. They are available in tubular form for artificial blood vessels, and in sheet form for patches. The inner surface of the prosthetic material composed only of expanded PTFE, which is adapted to contact blood, is gradually covered after implantation with a false membrane spreading from the stitched region to form a structure which is comparable to the inner surface of a natural blood vessel in thrombosis resistance. The fibrils in a baked product are somewhat more likely to cause thrombosis than in an unbaked product. Both the baked and unbaked products have so smooth surfaces that the false membrane formed thereon is likely to peel away when some external impact has been applied thereto. If any such structure formed newly on the prosthetic material or any other matter adhering thereto peels away, the fragments thereof are likely to block finer blood vessels, resulting in serious thrombosis.
The outer surface of such prosthetic material, which is adapted to contact the body tissues, is also so smooth that no satisfactory adherence of the body tissues, or growth into the fine structure of the tissues takes place after implantation. Various kinds of prosthetic materials composed of expanded PTFE, and woven or knitted material have been proposed to improve the compatibility of expanded PTFE with the body tissues, and prevent the material from being torn by the stitching thread. These composite prosthetic materials are so reactive with the body tissues that they may cause thrombosis, or prevent the formation of a uniform false membrane. What is worse, serious thrombosis may result from the separation of fibers from the knitted material in the stitched region.
The smooth surface of expanded PTFE is difficult to dye. This is a great disadvantage of expanded PTFE for use as clothing material. It is not suitable for use as a facing for a laminate, but is mainly used to form an intermediate layer therein. A velvety surface would certainly give expanded PTFE a wider range of application in the field of clothing.